The present invention refers to an electrical machine including a stator, which has magnetic flux conductors and a winding extending through said magnetic flux conductors, and at least one movable element, which is movable in relation to the stator.
Linear electrical machines are known in various connections, for instance as free-piston engines, i.e. an engine with a piston, which is mechanically freely movable in a cylinder housing. Consequently, there is no mechanical element, such as a connecting rod or a crank shaft, for the transfer of energy from the piston, but the kinetic energy of the piston is transformed directly to electrical energy by means of the linear electrical machine. Such a linear generator could, for instance, include a magnetic element in the piston and an electromagnetic element in the wall of the cylinder housing.
U.S. Pat. No. 5,788,003 discloses such a combustion engine of a free-piston type for driving a motor vehicle. The engine co-operates with an integrated electric generator, wherein the piston, which has an inner electrically conducting winding, is arranged to perform a reciprocating movement in relation to a surrounding outer winding. During operation, the outer winding produces a magnetising field, wherein electric effect is generated in the inner winding. The generated electric effect is obtained from the inner winding by a mechanical commutator, wherein the electric effect then may be used by an electric motor for driving the vehicle. Such a commutator is subjected to wear and has a limited life time.
U.S. Pat. No. 5,347,186 refers to a linear electric generator in which a magnet and a winding are provided to move in relation to each other. The relative movement of the winding in the magnetic field generates a current in the winding, which may be used by an external current circuit. This document describes the basic principles for magnetic induction, a phenomenon which has been known since very long ago. The new matter appears to be a member, which is intended to maintain a neutral position for the relative movement.
Conventional electrical machines operate according to the longitudinal flux principle, which means that the magnetic flux plane of each stator element is parallel to the moving direction of the rotor. SE-B-463 061 discloses an electrical machine, which operates according to the transversal flux principle. This known machine includes a rotor with a number of permanent magnets and a stator with a corresponding number of stator elements, which are provided in such a manner that the induced magnetic flux mainly follows a path perpendicular to the rotary direction of the rotor. The known machine is characterised by a high effect density, i.e. a high effect in relation to the volume of the machine is obtained. The document discloses rotary machines with a transversal flux. Certainly, a linear design is disclosed in the document but merely for an illustrating purpose. The document gives no information about how the transversal principle can be transferred to linear machines, and in particular not, to linear machines with an element describing a reciprocating movement in relation to the stator.
DE-A-198 11 075 discloses a linear electric machine having a stator with a plurality of magnetic flux conductors and a movable element with a number of permanent magnet elements. A closed winding path extends substantially in parallel to the moving direction of the movable element. Each magnetic flux conductor forms together with a permanent magnet element a closed magnetic flux circuit. The direction of the magnetic flux is the same in each circuit along the movable element, and thus the magnetic flux conductors are provided at a distance from each other, which corresponds to the width of a permanent magnet element.
The object of the present invention is to provide an improved electrical machine with a stator and a reciprocating movable element. In particular, it is aimed at such a machine with a high efficiency.
This object is obtained by an electrical machine including at least one first machine unit, which includes
a stator, which includes a plurality of magnetic flux conductors and an electric conductor forming a winding extending in a substantially closed winding path through each magnetic flux conductor, and
at least one first movable element, which includes a number of permanent magnet elements and which is movable in a reciprocating movement in relation to the stator along a first movement path in a space having a finite length and being formed by at least some of said magnetic flux conductors,
wherein the substantially closed winding path includes a first current carrying portion, which extends substantially in parallel with the movement path,
wherein each magnetic flux conductor is arranged to form, together with one of said permanent magnet elements, a closed magnetic flux circuit extending around said current carrying portion, and
wherein the magnetic flux conductors are arranged in an alternating order with respect to the direction of the magnetic flux in relation to the permanent magnet elements in the respective magnetic flux circuit.
By such a design, an electrical machine is obtained, which operates according to the transversal flux principle and which includes a reciprocating movable element that advantageously may form or be fixedly connected to a piston of a mechanical machine such as an engine or a pump. Thanks to the extension of the winding in a substantially closed winding path through each magnetic flux conductor of the stator, the total quantity of the winding may be reduced, which leads to small losses. Furthermore, the magnetic flux conductors of the stator may be arranged relatively closed to each other, which provides a high effect density with regard to the weight as well as the volume of the electrical machine. In such a way, a high efficiency of the electrical machine may be obtained.
Thanks to the alternating order, it is ensured that the voltage induced in the winding at each point of time has the same direction, and that the voltage through the winding changes direction simultaneously for each magnetic flux circuit when the movable element moves in such a way that the permanent magnet elements moves one step with regard to the magnetic flux conductors.
According to an embodiment of the invention, each magnetic flux circuit includes a magnetic flux, which is in parallel with a plane extending substantially perpendicularly to the movement path. The distance between a centre point of adjacent permanent magnet elements may thereby be substantially equal to the distance between a centre point of adjacent magnetic flux conductors. In such a way, the electrical machine according to the invention will, along a certain length of the stator, include the same number of stator elements, i.e. magnetic flux conductors, as permanent magnet elements of the movable element. Consequently, a high effect density of the electrical machine is obtained. Such a design may be provided by arranging the permanent magnet elements in an alternating order with respect to the magnetic direction of the permanent magnet elements.
According to a further embodiment of the invention, each magnetic flux circuit includes a first gap between a pole of the permanent magnet element of the circuit and an end surface of the magnetic flux conductor of the circuit, and a second gap between a second pole of the permanent magnet element of the circuit and a second end surface of the magnetic flux conductor of the circuit. The permanent magnet elements may thereby be arranged in such a way that the magnetic direction extends substantially perpendicularly to the moving direction of the movable element in the movement path. In such a manner, it is possible to design the movable element with at relatively small width and height since the movable element does not require any substantial further components. The movable element may thus be given a low weight, which is an important advantage for a movable element describing a reciprocating movement.
According to a further embodiment of the invention, the movable element is connected to at least one piston, which is movably arranged in a housing. The electrical machine may in such way be utilised as an electrical generator, wherein the piston is moved in the housing by means of a combustion process in a manner known per se. The electrical machine may also be utilised as an electrical engine for driving a piston pump, for instance.
According to a further embodiment of the invention, the first machine unit includes a second movable element, which includes a number of permanent magnet elements and which is movable in a reciprocating movement in relation to the stator along a second movement path extending in parallel with the first movement path, and wherein the substantially closed winding path includes a second current carrying portion, which extends substantially in parallel with the second movement path. In such a way, a very large part of substantially closed winding path may be utilised for current generation, and thus the losses are kept at a very low level. Preferably, also the second movable element is connected to the piston. However, it is to be noted that the two movable elements may be connected to a respective piston, or more precisely to a separate piston at each end of the movable element. Furthermore, the first current carrying portion of the winding path is associated substantially with a first half of said magnetic flux conductors, and the second current carrying portion of the winding path is associated substantially with a second half of said magnetic flux conductors. Preferably, the permanent magnet elements of the first movable element are arranged to co-operate with the magnetic flux conductors that are associated with the first current carrying portion, and the permanent magnet elements of the second movable element are arranged to to co-operate with the magnetic flux conductors that are associated with the second current carrying portion.
According to a further embodiment of the invention, adjacent permanent magnet elements of said movable elements are separated from each other by an intermediate element, which is substantially magnetically non-conducting. Thereby, the permanent magnet elements and the intermediate elements of the movable element may be arranged to form a structure, which resists the forces acting on the movable element during the use of the electrical machine. Such a design may preferably have the shape of an elongated rod extending between and connecting two pistons. Such a rod may thus be constructed from merely permanent magnet elements and intermediate elements in an alternating configuration. Moreover, adjacent magnetic flux conductors of the stator may be separated from each other by an intermediate element, which is magnetically isolating.
According to a further embodiment of the invention, the electrical machine also includes a second machine unit, which includes
a stator, which includes a plurality of magnetic flux conductors and an electric conductor forming a winding extending in a substantially closed winding path through each magnetic flux conductor, and
at least one first movable element, which includes a number of permanent magnet elements and which is movable in a reciprocating movement in relation to the stator along a first movement path in a space having a finite length and being formed by at least some of said magnetic flux conductors,
wherein the substantially closed winding path includes a first current carrying portion, which extends substantially in parallel with the first movement path,
wherein each magnetic flux conductor is arranged to form, together with one of said permanent magnet elements, a closed magnetic flux circuit extending around said portion, and
wherein the first machine unit is arranged to operate in a first phase position and the second machine unit is arranged to operate in a second phase position displaced from the first phase position by a phase angle.
By such a further machine unit, the electrical machine may be designed as a two phase machine, wherein the two machine units may be displaced from each other by a suitable phase angle, for instance 90xc2x0. It is to be noted, that it is of course possible to design the electrical machine with several phases, for instance three phases, wherein the electrical machine includes three such machine units. The different machine units may be arranged beside each other in such a way that the movable elements are moving in parallel to each other. Thereby, it is possible to connect the movable elements of the different machine units to one or two common pistons or two separate pistons for each machine unit.